A novel and compact ultra-wideband (UWB) bandpass filter (BPF) based on the broadband balun mechanism is proposed. The proposed structure is made up of two back-to-back open circuited meandered microstrip lines on the top plane coupled in a broadside fashion with a short circuited co-planar waveguide in the ground. This configuration gives rise to a smooth UWB passband with an insertion loss of <0.19 dB in simulation and two transmission zeros at the lower and upper passband edges which provide sharp selectivity. To confirm the theory a prototype is fabricated and its measured response is found to be in close agreement with the simulated response. The proposed structure is very small in size measuring only 14.6 × 7.3 mm2. © The Institution of Engineering and Technology 2015.